One continuing goal of semiconductor device application is to increase the level of device integration, or in other words to increase the density of devices across a supporting substrate. Methods for increasing the density can include decreasing the size of individual devices and/or increasing the packing density of the devices (i.e. reducing the amount of space between adjacent devices). In order to develop higher levels of integration it is desirable to develop new device constructions which can be utilized in semiconductor applications and to develop new methods of fabricating semiconductor device constructions.
A relatively common semiconductor device is a memory device with a dynamic random access memory (DRAM) cell being an exemplary memory device. A DRAM cell comprises a transistor and a memory storage device with a typical memory storage device being a capacitor. Modern applications for semiconductor devices can utilize vast numbers of DRAM unit cells.
Transistor structures comprise a channel region between a pair of source/drain regions, and a gate configured to electrically connect the source/drain regions to one another through the channel region. The transistor constructions utilized in semiconductor constructions will be supported by a semiconductor substrate. The semiconductor substrate will have a primary surface which can be considered to define a horizontal direction. Transistor devices can be divided amongst two broad categories based upon the orientations of the channel regions relative to the primary surface of the semiconductor substrate. Specifically, transistor structures which have channel regions that are primarily parallel to the primary surface of the substrate are referred to as planar transistor structures, and those having channel regions which are generally perpendicular to the primary surface of the substrate are referred to as vertical transistor structures. Since current flow between the source and drain regions of a transistor device occurs through the channel region, planar transistor devices can be distinguished from vertical transistor devices based upon the direction of current flow as well as on the general orientation of the channel region. Specifically, vertical transistor devices are devices in which the current flow between the source and drain regions of the devices is primarily substantially orthogonal to a primary surface of a semiconductor substrate, and planar transistor devices are devices in which the current flow between source and drain regions is primarily parallel to the primary surface of the semiconductor substrate.
There is continuing interest in the development of methodologies by which vertical transistor devices can be incorporated into integrated circuitry applications due to, among other things, advantages in packing density that can be obtained utilizing vertical transistor devices relative to planar transistor devices. Vertical transistors can also help alleviate problems associates with leakage current.
Leakage current can be a significant concern and problem in low voltage and low power battery operated circuits and systems and particularly in DRAMs. Where low voltages are used for low power operation there can be a problem with threshold voltages and stand by leakage current. Small threshold voltage magnitudes are utilized to achieve significant overdrive and reasonable switching speeds but can result in large sub-threshold leakage current. Various device structures have been developed to provide some improvement in sub-threshold leakage current characteristics. Many of the developed structures, including vertical transistor structures which can reduce leakage current can be complicated and/or expensive to produce. Difficulties are frequently encountered in attempting to produce the vast arrays of vertical transistor devices desired for semiconductor applications while maintaining suitable performance characteristics of the devices. It would therefore be desirable to develop new semiconductor device constructions applicable for utilization in DRAM structures and to develop new methods for fabricating vertical transistors and DRAM structures.